The modification of polystyrene with butadiene-styrene copolymer via bulk polymerization is taught by U.S. Pat. No. 3,178,489. Such polymerizations are conducted by dissolving the rubber in a mixture of the monomers to be polymerized and the patent discloses that the polymerization can be conducted in various known bulk polymerization equipment including kettles, columns or towers, extruders or any other equipment where a viscous polymerization mass can be maintained at controlled elevated temperatures. Polymerizations are conducted in two stages, a first stage in a kettle carried out between 90.degree. to 120.degree. C. and a second stage in a tower between 130.degree. and 200.degree. C. There is no clear teaching of the manner in which an extruder can be employed.
U.S. Pat. No. 3,536,680 provides a method for polymerizing ethylenically unsaturated monomers in an extruder, although there is no disclosure of rubber modification. Polymerization is conducted within the extruder between 30 seconds to 30 minutes and temperatures disclosed range from 100.degree. to 400.degree. C. resulting from kneading and shearing action. Although the use of the twin screw extruder disclosed in the patent is relevant, the temperatures are too high for the formation of rubber-modified plastic resins and, the time is longer than is desirable.
A U.S. patent which provides for the continuous production of copolymers from 1,3-dienes and monovinyl aromatics is No. 3,780,139. The temperature within the extruder is lower than the aforementioned patent viz., 50.degree. to 150.degree. C. and residence time ranges from about 10 to 30 minutes. Nevertheless, the patent does not disclose the formation of resin copolymers of monomers such as styrene, acrylonitrile and methacrylate grafted onto a preformed elastomer.
Therefore, the prior art of which we are aware has not disclosed a satisfactory process for the preparation of high impact strength rubber-modified thermoplastic resins of the ABS or high nitrile type which resins have improved processing properties over comparable resins produced via emulsion polymerization.